As is known in the art, wind energy can be converted to electrical energy with a wind turbine generator (WTG).
Virtually all wind turbines are based on one of the three main wind turbine types:                fixed speed WTGs with a direct grid-coupled (asynchronous) squirrel cage induction generator;        variable speed WTGs with a doubly fed induction generator; and        variable speed WTGs with a direct-drive synchronous generator.        
A fixed-speed WTG is typically connected to the grid through an induction (asynchronous) generator for generating real power. Wind-driven blades drive a blade rotor that in turn operates through a gear box (i.e., a transmission) to turn a gearbox output shaft at a fixed speed. The gearbox output shaft is connected to an induction generator for generating real power.
In the induction generator the rotor and its associated conductors rotate faster than the rotating flux applied to the stator from the grid (i.e., higher than the synchronous field frequency). The difference in these two values is referred to as “slip.” At this higher speed, the direction of the induced rotor current is reversed, in turn reversing the counter EMF generated in the rotor windings, and by generator action (induction) causing current (and real power) to be generated in and flow from the stator windings.
The frequency of the voltage generated in the stator is the same as the frequency of the voltage applied to the stator to develop the stator excitation. The induction generator may use a capacitor bank for reducing reactive power consumption (i.e., the power required to generate the initial stator flux) from the power system.
The fixed-speed wind turbine is simple, reliable, low-cost and proven. But its disadvantages include uncontrollable reactive power consumption (as required to generate the stator rotating flux), mechanical stresses, limited control of power quality and relatively inefficient operation. In fact, wind speed fluctuations result in mechanical torque fluctuations that can result in fluctuations in the electrical power on the grid.
Variable speed WTG operation can be achieved only by decoupling the electrical grid frequency and the mechanical rotor frequency. The rotational blade speed of a variable speed WTG can be controlled to continuously adapt to the wind speed and maximize the power generated by the wind turbine. Since an electric generator is usually coupled to a variable speed WTG rotor through a fixed-ratio gear transmission, the electrical power produced by the generator has a variable frequency.
Decoupling the grid frequency from the rotor mechanical frequency requires use of an electronic power converter. Generally, the power converter imparts characteristics to the generated electricity that are required to match electricity flowing on the grid, including controllable active power flow, voltage magnitude and frequency regulation. Thus the converter converts the variable electrical frequency and voltage output from the generator stator to the grid frequency and voltage.
The power converter uses either a direct frequency converter or a full converter. The full converter first converts the variable frequency stator output to DC in a generator-side converter (rectifier). A grid-side or network-side converter (inverter) reconverts the DC to a fixed-frequency AC (equal to the grid frequency) for supplying to the electrical grid. In both cases, the power converter effectively electrically decouples the WTG from the electrical grid.
The AC output voltage of the network-side converter is filtered and supplied to the grid via a step-up transformer. Protective switchgear can be included to provide a reliable connection to the grid and to isolate the WTG and converter from the grid as required.
Although variable speed WTGs are advantageous from the perspective of increased energy conversion and reduced mechanical stresses, the electrical generation system is more complicated than that of a constant speed wind turbine due primarily to the need for a power converter.
In a variable speed turbine having a doubly fed induction generator (DFIG) the grid is fed directly from the stator windings. A power converter feeds the rotor winding, but a converter is not interposed between the grid and the stator windings. The electrical rotor frequency is varied by the converter, thus decoupling the mechanical and electrical frequency and making variable speed operation possible.
The exciting field produced by the generator's rotor rotates relative to the generator's rotor with a variable speed according to the wind turbine blade speed. The variable rotor speed is compensated by action of the converter that correspondingly adjusts the speed of the exciting field relative to the rotor's rotation speed. As a result, the sum of the two speeds, i.e. the speed of the exciting field relative to the rotor field is always a constant value equal to the fixed grid frequency. Note that the converter used in the DFIG wind turbine is used only to produce the variable frequency exciting-field rotor currents.
Both fixed speed and variable speed WTGs are designed to operate in parallel with a synchronous generator, both supplying power to the grid. The WTG's synchronize to the grid frequency to produce a constant frequency electrical output.
The fixed speed WTG requires synchronization to maintain a constant slip value and thereby supply power at the grid frequency.
A synchronizing frequency is also necessary for variable speed WTGs since they require a system frequency for use in switching/commutating the switching devices in the network-side converter to supply fixed frequency power to the grid.
If this synchronization with the grid is not provided, the WTGs operate in a so-called island mode, which results in degradation in the quality of the electricity supplied to the local load during the islanding period due to the lack of utility control of the WTG. During this time, uncontrolled voltage or frequency excursions can damage customer equipment connected to the WTG.
Also, if the island mode was caused by disconnection from the grid due to a transient system fault, when the system interrupting devices try to re-close the grid connection after a few cycles, the re-closing action can potentially damage the WTG. For example, damage may occur if the voltage in the island mode is not the same as the grid voltage. Also, when the grid is reconnected, the grid voltage can have a different phase angle with respect to the island voltage. This can cause a relatively large over-current excursion that can damage the WTG.
FIG. 1 illustrates a prior art wind turbine generator park 1 comprising variable speed wind turbine generators 2, 3.
The WTGs 2, 3 generate electrical power that is supplied to a power system or utility grid 37 via a node 35. Preferably, the WTGs 2, 3 are variable speed wind turbines, i.e., the rotational speed of their respective generator rotors is variable depending on wind conditions.
Each WTG 2, 3 comprises turbine blades 4, 5 attached to a rotor shaft 6, 7 for transmitting the torque of the wind-driven blades 4, 5 to a gearbox 8, 9. An output shaft of the gearbox 8, 9 drives an AC generator 17, 19 for transforming the mechanical power provided by rotation of the rotor shaft 6, 7 to electrical power. The gearbox 8, 9 provides a transmission ratio that allows the gearbox output shaft to turn at a different speed than the rotor shaft 6, 7. Preferably the gearbox output shaft turns at a speed that optimizes the electricity generated by the AC generators 17 and 19.
The AC generator 17, 19 can be either a synchronous generator or an asynchronous (induction) generator and further comprises power electronics components. Generally, in a synchronous generator, a generator rotor rotates at the same rotational frequency as the rotating magnetic field produced by a generator stator (or with an integer relationship to the frequency of the rotating magnetic field, where that integer relationship depends on the number of rotor pole pairs).
In contrast thereto, in an asynchronous generator (induction generator) the rotational frequency of the stator's magnetic field (conventionally 60 Hz when the stator magnetizing current is supplied from the electrical grid) is independent from the rotational frequency of the rotor. The difference in rotational frequency of the rotor and the stator is numerically described by a slip value.
If the generators 17, 19 of FIG. 1 comprise synchronous generators, the frequency of the output power therefrom depends on wind velocity. But that output frequency must be converted to the frequency of the electrical grid to which the generators 17, 19 supply electricity through the node 35.
The frequency conversion process is accomplished by action of power electronics frequency converters 21, 23. Each frequency converter converts the frequency of the electrical power delivered by generators 17, 19 into electrical power having a fixed frequency corresponding to the frequency of the power system 37. Each frequency converter 21, 23 comprises a respective generator-side converter (rectifier) 25, 27 for converting the AC current produced by the generator 17, 19 into a DC current. A network-side converter (an inverter) 29, 31 converts the DC current back to an AC current at the frequency of the power system 37. The AC output of the network-side converter 29, 31 is supplied to the power system 37 via the node 35 and a transformer 33.
As is known, the magnitude of power flow on a power system depends primarily on the voltage phase angle difference between two points, since a line reactance between the two points is constant and the voltage at each of the two points is nearly constant. Then the power flow is governed by the equation:P=(V1×V2/X12)sin δwhere V1 and V2 are the voltages at locations 1 and 2, X12 is the reactance between locations 1 and 2, and δ is the phase angle difference between the voltages at locations 1 and 2.
The same formula can be applied to determine the power that electric generators deliver to a section of a power system. In this application, V1 and V2 are the voltage magnitudes at the respective terminals of the generators 1 and 2, X12 is the line reactance between the two generators, and δ is the phase angle difference between the two generator voltages. Note that if a difference between the voltage phase angles of generators 1 and 2 is zero, no power is supplied to a load.
A wind turbine generator is frequency-synchronized with the system voltage as that voltage is determined or measured at the WTG terminals. The voltage at the WTG terminals has a phase angle and magnitude determined by the aggregate of generators and loads on the system.
According to the prior art, a phase-locked loop (PLL) of a WTG controller detects the phase angle of the network voltage present at its terminals. The network voltage is supplied by one or more synchronous generators on the grid. The WTG controller further establishes a WTG voltage at a given phase angle difference so that the WTG and the synchronous generators can provide real power to the grid according to the equation,P=(Vt×Vsys/X)sin δwhere P is the total power delivered to the grid by the WTG, Vt is the wind turbine generator converter output voltage (or the voltage at the WTG output terminals), Vsys is the grid or system voltage, X is the reactance of the line and transformer(s), if applicable, between the WTG and the power system or the grid, and δ is the phase angle by which the WTG voltage (as provided from the converter and supplied to the WTG output terminals and from there to the grid) leads the power system voltage.
Note that according to the equation immediately above, the WTG output and the system voltage are not “in phase synchronism” as the phase angles differ by the angle δ. In fact, if the phase angle between the WTG voltage and the voltage of the system to which the WTG is connected is zero, no power is transferred to system loads, i.e., sin δ=0 if δ=0.
However, as is known by those skilled in the art, synchronism generally refers to frequency synchronism. If the frequency of the WTG and the system is the same and the phase angle remains constant or varies only slowly, the WTG and the system are considered to be operating in synchronism. Then if the phase difference is constant the output power supplied to the grid is constant. If the phase difference varies slowly, the power supplied to the grid varies slowly at that same slow rate.